The invention relates generally to electron discharge tubes and particularly to tubes of the type wherein an electron image is intensified by passage through a channel plate electron multiplier.
Electron image intensifying tubes such as image tubes may include a channel plate electron multiplier. Such tubes are described in detail in, for instance, U.S. Pat. Nos. 3,260,876 and 3,487,258 both to B. W. Manley et al and U.S. Pat. No. 3,497,759 to B. W. Manley. A proximity focussed image tube generally includes a flat photocathode and a flat phosphor screen facing one another in an evacuated flat glass envelope. Spaced between the photocathode and the phosphor screen is a thin, flat channel plate whose faces are parallel to, and closely spaced from, the photocathode and phosphor screen. The channel plate has a large number of small, round, parallel channels extending from one face to the other. The channels are slanted at a bias angle of about 5.degree. with respect to the input and output faces of the plate. Both input and output faces are provided with a conducting electrode such as a coating of chromium. The inside surfaces of the channels are activated with hydrogen to increase secondary emission.
In operation of the tube, appropriate accelerating voltages are applied to the photocathode, the input electrode of the plate, the output electrode of the plate, and the phosphor screen, so that electrons from the photocathode strike the inside walls of the channels, are multiplied, while travelling through the channels exit at the output face of the plate, and strike the phosphor screen to produce a visible output image.
Increased spacing of the output screen from the output electrode of the plate permits a higher accelerating voltage to be applied between the output electrode and screen, and thus further increases the intensity of the output image without increasing risks of charging of the tube walls or causing high field breakdown. However, such increased spacing results in decreased resolution since the beam of electrons from individual channels rapidly spreads as it leaves the output face of the plate. Beyond a short distance, on the order of mils, beams from adjacent channels overlap. Such decreased resolution has, nevertheless, been avoided by endspoiling of secondary emission near the channel end at the output face to effectively decrease the output aperture of the channels. In previous devices, endspoiling is provided by extending the output electrode metallizing a uniform short distance into the end channel so that no multiplication can occur near the output face. The endspoiling in effect collimates each of the output beams from the channels.
The detection efficiency of the input image can also be increased by increasing the bias angle of the channels. Increase in bias angle also results in increased multiplication since there are then a larger number of electron impacts near the input of the channel. However, slant of the channels with respect to the output phosphor screen, results in astigmatism. That is, the output beam from each channel strikes an elongated spot on the output screen rather than a round spot, much as a round light beam impinging on a surface at an angle illuminates an elongated spot. The astigmatism decreases resolution with increasing bias angles of the channels and with increased spacing of the output screen from the channel plate.